1. Field of the Invention
This invention relates to vehicles having selective load-bearing and non-load bearing axles. In one of its aspects, the invention relates to a means for controlling the load-bearing relationship of a liftable tandem axle in a vehicle.
2. Prior Art
Vehicles having a body located on a vehicle frame can have a plurality of rear wheel-bearing axles. These rear axles act to support the rear of the vehicle. Because the body is located generally toward the rear of the vehicle, and because the body can often be loaded with heavy loads, the rear axles must be suitable for supporting heavy loads. The use of multiple rear axles generally serves to distribute the load over the multiple axles, thereby reducing the load which must be supported by each individual rear axle.
Vehicles can include a liftable rear axle. Examples of liftable axle mechanisms are disclosed in the U.S. Pat. No. to Pierce, et al, 3,771,812, issued Nov. 13, 1973 and the U.S. Pat. No. to Taylor, 4,293,145, issued Oct. 6, 1981. Typically, these axles are provided with a suspension system between the axles and the frame. Suspension systems include a rigid trailing arm pivotably mounted to a frame bracket and an air spring between the trailing arm and the frame. The rear axle can be lowered to a ground-engaging position when the body is loaded or lowered so that the liftable axle, together with the other rear axle(s) supports the weight of the loaded body. The liftable axle can be lifted into a raised, non-ground engaging or non-load-bearing position when the body is unloaded or raised. The raising of the liftable rear axle into a raised position, when the vehicle body is empty, increases fuel efficiency and reduces tire wear.
Federal, sate or local regulations may require that the liftable rear axle be raised into a non-ground-engaging or non-load-bearing position only when the body is in an empty or raised condition. Conventional systems require the vehicle operator to manually control the positioning of the liftable rear axle. The operator may err and leave the rear, liftable axle in a raised or unloaded position even though the vehicle body is loaded or lowered. Thus, the operator's failure to lower the liftable rear axle into a ground-engaging or lowered position may cause the vehicle to be in violation of applicable regulations. The operator's failure to lower the liftable axle into a ground-engaging or load-bearing position when the body is loaded or lowered can also cause the remaining rear axle(s) to support an increased proportionate amount of the weight of the body. Thus, the remaining rear axle(s) can be loaded to an excessive degree, decreasing the life of the remaining rear axle(s) and decreasing the safety with which the vehicle can be operated.